"I eat the apple."
Translation:Ich esse den Apfel.
Sometimes it is easier if you make two seperate 'lists', subject = der/die/das; object = den/die/das. Identify the word as the subject or an object, then go from there. Since the word is an object in this case you would go to that list and you're only options are den/die/das. I know it helped me.
There are certain nouns which always take a particular gender - words ending with 'chen' or 'lein' always = das (neuter); words ending with 'heit', 'keit', 'ung' or 'schaft' always = die (feminine). Most words related to days/months/seasons etc = der (masculine) and most profession/job words have the der-form as their 'base' and then have a specific die-form (i.e. der Lehrer, die Lehrerin).
However, there are lots of words that don't have these endings/rules and, as the other commentors have said, you have to learn the article/gender with the noub.
Verbs congigate differently depending on your subject. Look at the ending of words for a better idea. Ich - e, Du - st, ect. Essen is weird because of the 'Du' form so look at Lieben (love). It goes like this... You drop the en from the end and add the correct ending. Ich liebe, Du liebst... Like that. If you followed this with Essen it would be Du essst and that just doesn't work, so the to compact the 'sss' it changes to vowel sound from the e to the i.
Dar isn't a choice here. I think you mean der Apfel and if that's the case its all in how you use it. If the apple is the subject you would say der (der apfle ist gut). If the subject is yourself and the apple is just an object then you would use den (ich esse den apfel). Thing about it like this... What is the sentence about? In this case it is about you and the apple is just an object. If you get confused like I do sometimes think about this... What is a complete sentence (not a command but a statement) "I am eating" or "am eating the apple". You can take out apple and still have a complete scentence so its just an object. English is the hardest language by the way lol
It doesn't matter if you are masculine... It's if the word is. What is the sentence about? If it is about 'the man', 'the woman', or 'the dog' (just using these words as examples) you would use der/die/das. If the sentence is about something else you would use den/die/das. Think about them as two seperate lists - one for subject and one for an object. The only one that changes is der to den. But if you think about them seperate its easier. "I love the... " however you finish that scentence it is about me so you know you will use den/die/das. Ich liebe den Mann. Ich liebe die Frau. Ich liebe das Hund. If you make them into two seperate lists, when you identify it as the object and go to look at your options then der won't even be an option for you to pick. I hope that helps.